Much effort has been expended in the study of adipose tissue - its structure, physiology, size and number of adipocytes and their response to hormones, etc., but relatively little attention has been paid to another aspect of human obesity, namely the increased lean weight which characterizes the majority of patients with this condition. In this regard human obesity differs from certain types of experimental and genetic obesity in animals who tend to have a reduced lean weight. Very little is known about the cause(s) of this increase in lean weight, nor is it known which component - lean or fat - is responsible for some of the metabolic abnormalities seen in obese individuals. We propose to make the following measurements in obese and non-obese subjects matched as closely as possible for age and height and sex: total body water, extracellular fluid volume, 40K assays, urinary excretion of creatinine and 3-methylhistidine (on meat-free diet), BMR, estimates of skeletal size and of muscle size (by CAT scan), and estimates of cardiac and visceral size and of adipocyte number and size. In this way we hope to better characterize the obese human, and hopefully to distinguish metabolically the minority who do not show an increase in lean weight from the majority who do. The result should be a more complete description of the obese state in man.